Significant design efforts have been directed lately to the task of providing a centralized lighting system whereby a single source of high intensity light could be utilized for the illumination of a number of separate remote locations requiring specific levels of light. Such efforts have led to proposals of central lighting systems for areas such as automotive lighting, display lighting for merchandise for instance and for home use. An example of such a central lighting system applicable in the automotive field can be found in U.S. Pat. No. 4,958,263 issued to Davenport et al. on Sep. 18, 1990 and assigned to the same assignee as the present invention. The goal of this automotive lighting system as well as any central lighting system is to achieve the most efficient light coupling between the light source and the light distribution arrangement and then, the most efficient delivery of the light output from the light distribution arrangement which is typically an optical fiber, light guide or equivalent. One example of an attempt to maximize light delivery at a particular point can be found in French Patent No. 1,383,413 issued to Karl Storz for a device for illuminating operating fields. This invention uses a pair of spaced apart reflectors on either side of an incandescent light source to collect light in a more efficient manner for use during surgical procedures. For an automotive application or any situation where space considerations are of significant importance in the design of a centralized lighting system, it would be advantageous to utilize components such as light guides, coupling elements and the actual light source which are of such a reduced size so as to allow for reduced space requirements thereby proving advantageous for an aerodynamically tapered appearance of the front end of the vehicle. Specifically, for a central lighting system to be acceptable from an economical point of view, the size of the light guide should be on the order of 6 mm or less in diameter to allow for the maximum system design flexibility. In an automotive application, present designs require a light guide with a diameter of approximately 10 mm in order to provide a sufficient amount of light output for a headlamp type of application. Known techniques allow for an intensity level of approximately 10 lumens per square millimeter which, using a 10 mm light guide, would result in approximately 785 lumens available for delivery to the road surface. Though this is an acceptable amount of light output, it is achieved using a light guide which is large, of a comparatively significant weight and lacking in flexibility relative to a light guide having a diameter of 6 mm or less. It would therefore be advantageous to provide a light coupling arrangement which could achieve a higher energy density so as to allow for a smaller diameter light guide.
The advantage of using a reduced diameter light guide can be appreciated when one considers that such small diameter light guide provides a higher lumen density measured in terms of lumens per unit of cross-sectional area of the light guide. In order to provide the most efficient light delivery system, substantially all of the light from the image of a high brightness light source must be introduced to the input face of the light guide. For the present invention, a light source having a flux measurement in excess of 4000 lumens delivered from a discharge lamp having an arc gap of less than 4 mm was utilized, such light source being available from General Electric Lighting in the form of the Model LE60, Light Engine.TM. high brightness light source. To achieve the desired efficiency using a smaller diameter light guide would require that the magnification of the image formed by a reflector associated with the light collection arrangement be reduced, preferably to unit magnification. It is known in the prior art that unit magnification of the light source can be achieved by using a spherical reflector in place of an elliptical one. U.S. Pat. No. 4,956,759 issued to Goldenberg et al on Sep. 11, 1990 describes an illumination system that utilizes a spherical reflector to form a unit magnification image. In such a system, the light source is positioned near the center of curvature of the reflector and an image of unit magnification is formed at a point near the source.
It can be further appreciated that for an automotive application, particularly the forward lighting requirements, it would be advantageous to provide a central lighting arrangement that could provide for illumination of both sides of a vehicle from a single light source. It would also be advantageous if such single light source, dual light output arrangement could be achieved with the highest coupling efficiency possible, for instance, an efficiency in excess of approximately 60 to 70 percent of the available lumen output of the light source. Using known light coupling techniques involving a central light source and an array of light guides typically yields a coupling efficiency on the order of approximately 40 percent. Therefore, by more efficiently coupling a larger percentage of the light source through the light guides, more light available for other purposes such as rear lighting of a vehicle, as well as other interior vehicle lighting. Of course, such light coupling efficiency would also provide a significant advantage to other application areas such as display lighting for merchandising for example.
It is a further requirement of such a central lighting system that light collected from the light source by the associated reflector arrangement provide the light input to the light guide at a small enough angle so as to provide for sufficient light capture at the input face of the light guide. In other words, in order to utilize a light guide having a numerical aperture (NA) value of 0.5, it is necessary to introduce the light to the input face of the light guide at an angle of approximately 30 degrees or less. In a typical light collection arrangement using a nearly hemispherical ellipsoidal reflector, the angle of entry of the light to the input face is on the order of greater than 50 degrees. Although it would be possible to utilize an optical fiber having a larger numerical aperture so as to allow for a larger input angle entry of light, such an optical fiber is more costly and less able to accommodate bending as may be needed in an automotive forward lighting application for instance. Accordingly, it would be a benefit to a new central lighting system if a means could be provided that would reduce the angle of entry so as to allow for a more efficient capture of light at the input face of the light guide having a 0.5 NA or smaller and to provide more efficient transfer to the output end.